Industrial process industries often rely on energy sources that include one or more combustion processes. Such combustion processes include operation of a furnace or boiler to generate energy from combustion, which is then used for the process. While combustion provides relatively low-cost energy, its use is typically regulated and combustion efficiency is sought to be maximized. Accordingly, one goal of the process management industry is to reduce the production of greenhouse gases by maintaining maximum combustion efficiency of existing furnaces and boilers.
In situ flue gas analyzers are commonly used for monitoring, optimizing and/or controlling combustion processes. Typically, these analyzers employ a zirconium oxide sensor to measure excess oxygen in the flue gas. Such an oxygen sensor is known and uses a principle of operation based on the Nernst equation. This principle of operation requires that the sensor cell be maintained at an elevated operating temperature. Typically, such elevated temperatures are achieved using a heater that is powered by an analyzer's electronics. In situ flue gas analyzers are particularly advantageous because they have no moving parts or sampling apparatus resulting in an extremely reliable probe that requires very little maintenance.
Some operators of combustion processes have applications that involve hazardous gases in the process itself or in the ambient gases in the area where the analyzer's electronics are installed. These operators are concerned that the cell heater may serve as a source of ignition to these hazardous gases inside the process or that the electronics can provide ignition to hazardous process or ambient gases that may be present. As a result of these concerns, these users must purchase analyzers with costly protection features.
When hazardous gases are present, there are typically two ways in which protection can be provided: explosion-proof enclosures and/or intrinsically safe circuitry.
When electronics are housed within explosion-proof enclosures, such enclosures can prevent the gases from entering the internal chamber of the enclosure. Additionally, if such gases do enter the enclosure and cause an explosion, the flame will not be able to propagate outside of the enclosure. One example of an explosion proof rating is an ATEX certification to EEx d IIB T6 standards EN50015 and EN50018 for potentially explosive atmospheres Parts 1 and 5.
The other protection scheme is to make the electronics intrinsically safe. When the electronics are intrinsically safe, they inherently cannot generate the required temperature or spark to generate an explosion, even under fault conditions. An example of an intrinsic safety specification is the standard promulgated by Factory Mutual Research in October 1998 entitled APPROVAL STANDARD INTRINSICALLY SAFE APPARATUS AND ASSOCIATED APPARATUS FOR USE IN CLASS I, II, AND III, DIVISION 1 HAZARDOUS (CLASSIFIED) LOCATIONS, CLASS NUMBER 3610. Intrinsic safety requirements generally specify such low energy levels that compliance is simply not possible with circuitry that involves high voltages, high currents, and/or high wattage, such as AC circuits.
One particular device that has been used in the past for explosive environments is sold under the trade designation Model 5081FG Two-Wire In Situ Oxygen Analyzer, available from Emerson Process Management. This device utilizes a zirconium oxide sensor to measure excess oxygen in combustion processes. However, the analyzer eliminates the use of a cell heater and instead uses high process temperatures to heat the zirconium oxide sensor cell to the temperature required by the Nernst equation for operation. The analyzer's electronics are intrinsically safe, and can be powered by 4-20 mA signal wires. While the Model 5081 FG has proved useful for measuring oxygen in or proximate hazardous locations. its use has been limited to applications that generate enough process heat to maintain the zirconium oxide sensor at the requisite elevate temperature. Additionally, when process heat is required for sensor operation, useful sensor data is not available until the process has sufficiently heated the sensor. Thus, the ability to measure oxygen during system startup has been limited for in situ oxygen probes that have intrinsically safe output.
Providing an in situ oxygen probe with an intrinsically safe output that could function at lower temperatures and/or during system startup would increase the applications to which such devices could be applied.